Modern diesel engines require exhaust gas recirculation (“EGR”) to the engine air intake to comply with engine exhaust regulations for NOx emissions. Methods and apparatus for exhaust gas recirculation are known, and typically divert a portion of gas exhausted from the cylinders, filter and cool it, and mix it into the intake charge air. Cylinder-to-cylinder EGR distribution is important for achieving compliance with exhaust emission regulations, maintaining acceptable engine operation, and to operating with optimal brake specific fuel consumption (“BSFC”). Thorough mixing of EGR with incoming charge air is required to meet these criteria. Current EGR mixer designs struggle to meet these requirements across all engine operating speeds.
Currently known solutions include, for example, mixers based on the venturi principle, as shown in U.S. Pat. No. 6,343,594 to Keoslin et al., and U.S. Pat. No. 6,267,106 to Feucht. Other known mixers disclose tubes that introduce exhaust gas in the inlet air stream direction, including U.S. Pat. No. 6,425,382 to Marthaler et al., U.S. Pat. No. 6,672,292 to Fischer, and U.S. Pat. No. 6,889,673.
U.S. Pat. No. 6,427,671 to Holze et al. shows a tube having a slanted opening with a truncated tip. The wall portion of the tube divides an inlet air flow, which rejoins on a downstream side of the tube to mix with incoming exhaust gas.
U.S. Pat. No. 5,492,104 to Elder et al. discloses an exhaust gas mixing tube mounting for a gasoline engine.
FIG. 4 shows an exhaust gas mixing tube 10 owned in common with the present invention. The mixing tube 10 of FIG. 4 has a cylindrical first part 12 transitioning into a curved, scoop-shaped guide wall 14 that curves into the direction of flow and flares outward laterally (relative to the tube axis) to direct exhaust gas into an inlet air flow.
FIG. 5 shows another mixer tube 16 owned in common with the mixing device of the invention. The mixer tube 16 has a cylindrical first part 18 followed by an elbow 20 bending about 90° relative to an axis of the first part and terminating in a mouth 22 to direct exhaust into an inlet air flow in the direction of the inlet air flow.
A device in accordance with the invention for mixing exhaust gas in an engine air intake includes a mixing chamber disposed or formed in an air intake conduit and a mixer tube to introduce exhaust gas into the mixing chamber. The mixer tube is connected to a source of exhaust gas to be recirculated to the engine.
The mixing chamber has an inlet for receiving intake air and an outlet for exhausting the intake air, and having an aperture or port between the inlet and the outlet, and, a mixer tube extending into the aperture, an end portion of the mixer tube disposed in the mixing chamber shaped as a cylindrical-section having an elongated bevelled opening facing the mixing chamber outlet and a wall facing the mixing chamber inlet. Exhaust gas enters the engine air intake flow in the mixing chamber through the elongated opening.
According to another aspect of the invention, an edge of the wall of the end portion of the mixer tube defining the elongated opening is oriented at not less than about 50° to the longitudinal axis of the mixer tube.
According to a preferred embodiment of the invention, a cross-sectional area of the end portion of the mixer tube is at least 40% of a cross sectional area of the mixing chamber at a point where the mixer tube extends into the mixing chamber. The wall of the mixer tube forms a paddle that causes the intake air to flow around the wall, facilitating exhaust gas entering the mixing chamber to mix with the charge air.
At low engine speeds and associated low intake air flows, pressure pulsations cause the charge air flow to reverse direction. The elongated opening provides a backflow mixing volume that collects the reverse flow and allows the air to mix with incoming exhaust gas.
According to the invention, the elongated opening of the mixer tube extends substantially a length of the mixer tube disposed in the mixing chamber.
According to another aspect of the invention, the edge of the wall of the end portion of the mixer tube defining the elongated opening is formed to be substantially elliptical. The substantially elliptical opening forms a curved tip at the end of the mixer tube.
According to another aspect of the invention, the tip of the mixer tube is spaced from a wall of the mixing chamber opposite the aperture by a distance not more than 20% of a width of the mixing chamber.
The tip of the mixer tube provides an edge that with the narrow spacing of the tip from the mixing chamber wall produces vortex shedding in the air flow during high air flow rates. The vortex shedding in the air flow helps mix charge air with the exhaust entering through the mixer tube opening.
According to yet another aspect of the invention, the mixer tube wall at least at the tip has a thickness that is not more than about 10% of a tube diameter.